An electrical deionization apparatus is an apparatus in which concentration compartments and deionization compartments are formed by arranging cation exchange membranes and anion exchange membranes between electrodes, i.e. a cathode and an anode, and taking a potential gradient as a driving force, ions in water to be treated (raw water) in the deionization compartments are made to migrate, and hence be separated out, through the ion-exchange membranes into the concentration compartments, thus removing ionic components.
FIG. 1 shows a conceptual drawing of a typical conventional electrical deionization apparatus. In the electrical deionization apparatus shown in FIG. 1, anion-exchange membranes A and cation-exchange membranes C are arranged alternately between a cathode (−) and an anode (+), thus forming deionization compartments and concentration compartments. By further repeating the alternate arrangement of anion-exchange membranes and cation-exchange membranes, a plurality of deionization compartments and concentration compartments are formed alternately. If necessary, ion exchangers are packed into the deionization compartments and the concentration compartments to promote migration of ions in the compartments. Moreover, the compartments contacting the anode and the cathode at either end are generally called an anode compartment and a cathode compartment, and have a function of giving and receiving electrons of an applied direct current.
In operation of such an electrical deionization apparatus, a voltage is applied between the anode and the cathode, and water is passed into the deionization compartments, the concentration compartments and the electrode compartments. Water to be treated having therein the ions to be subjected to treatment is fed into the deionization compartments, and water having a suitable quality is passed into the concentration compartments and the electrode compartments. In FIG. 1, an example is shown in which RO treated water is fed into all of the deionization compartments, the concentration compartments and the electrode compartments. When the water is passed into the deionization compartments and the concentration compartments in this way, in each of the deionization compartments the cations and anions in the water to be treated are attracted to the cathode and the anode respectively; because only anions selectively permeate through the anion-exchange membranes and only cations selectively permeate through the cation-exchange membranes, cations (Ca2+, Na+, Me2+, H+etc.) in the water to be treated pass through a cation-exchange membrane C and migrate into a concentration compartment on the cathode side, and anions (Cl−, SO42−, HSiO32−, HCO3−, OH− etc.) pass through an anion-exchange membrane A and migrate into a concentration compartment on the anode side. On the other hand, migration of anions from the concentration compartment on the cathode side into the deionization compartment and migration of cations from the concentration compartment on the anode side into the deionization compartment are blocked due to each of the ion-exchange membranes having a property of blocking ions of the opposite sign. As a result, deionized water having a reduced ion concentration is obtained from the deionization compartments, and concentrated water having an increased ion concentration is obtained from the concentration compartment.
According to such an electrical deionization apparatus, by using water containing few impurities, for example, RO (reverse osmosis membrane) treated water as the water to be treated, pure water of yet higher purity is obtained as the deionized water. Recently, there has come to be a demand for ultrapure water of yet higher purity, for example, ultrapure water for semiconductor manufacture. In recent electrical deionization apparatuses, a method is thus known in which cation-exchange resin beads and anion-exchange resin beads are mixed together and packed as an ion exchanger into the deionization compartments and/or the concentration compartments and/or the electrode compartments so as to promote migration of ions in these compartments. Furthermore, there has also been proposed a method in which as an ion exchanger, a cation-exchange fibrous material (nonwoven fabric etc.) on the cation-exchange membrane side and an anion-exchange fibrous material on the anion-exchange membrane side are disposed face-to-face in a deionization compartment, and a spacer or an ion-conducting spacer having ion-conductivity is packed between these ion-exchange fibrous materials (see, for example, JP-A-H5-64726, and WO 99/48820).
When water to be treated is passed into a deionization compartment having an ion exchanger packed therein as described above, ion-exchange groups in the ion exchanger and the salt to be removed in the water to be treated undergo an ion-exchange reaction, whereby the salt is removed. For example, in the case of using NaCl as the salt to be removed, sulfonic acid groups as cation exchange groups and a quaternary ammonium salt as anion exchange groups, removal of NaCl can be explained as follows:
When the water to be treated having the salt to be removed (NaCl) dissolved therein contacts the cation exchanger, the cations (Na+) in the water to be treated are subjected to ion exchange by the cation-exchange groups, and are adsorbed onto the solid phase (cation exchanger) and thus removed (equation 1).NaCl+R—SO3−H+⇄HCl+R—SO3−Na+  (1)
After the cations have been removed to some extent by contacting with the cation exchanger, the water to be treated next contacts the anion exchanger. At this time, the acid (HCl) that has been produced by the ion-exchange reaction (equation 1) due to the cation-exchange groups is completely neutralized as shown in equation 2.HCl+R—N+(CH3)3OH−→H2O+R—N+(CH3)3Cl−  (2)
On the other hand, salt to be removed in the water to be treated that has not reacted with the cation exchanger contacts the anion exchanger, and the anions (Cl−) are subjected to ion exchange by the anion-exchange groups as shown in equation 3, and are adsorbed onto the solid phase (anion exchanger) and thus removed.NaCl+R—N+(CH3)3OH−⇄NaOH+R—N+(CH3)3Cl−  (3)
Next, the water to be treated contacts the cation exchanger, and the alkali (NaOH) that has been produced by the ion-exchange reaction due to the anion-exchange groups (equation 3) is neutralized as shown in equation 4.NaOH+R—SO3−H+→H2O+R—SO3−Na+  (4)
Equations 1 and 3 above are equilibrium reactions, and hence the salt to be removed contained in the water to be treated is not completely removed by ion exchange upon contacting the anion exchanger once and the cation exchanger once, but rather remains in the water to be treated to some extent. To remove the ions efficiently, it is necessary to carry out the reactions of equations 1 to 4 repeatedly, and hence it is important to make the water to be treated contact the cation exchanger and the anion exchanger alternately as many times as possible so as to make the salt to be removed migrate into the solid phase through the reactions of equations 1 to 4.
For the ions to be removed in the water to be treated to undergo ion-exchange and neutralization reactions as described above, a two-stage process of the ions to be removed migrating into the vicinity of functional groups and then being subjected to the ion-exchange reaction is necessary. In an electrical deionization apparatus, the water to be treated is fed continuously into each deionization compartment, and must undergo the ion-exchange and neutralization reactions during the short time of passing through the deionization compartment; it is thus desirable for the ions to be removed in the water to be treated to diffuse into the vicinity of functional groups of an ion exchanger in a short time, and for the frequency of contact between the functional groups and ions to be kept high.
Moreover, in an electrical deionization apparatus, it is necessary for the ions to be removed that have been adsorbed onto the solid phase (ion exchangers) by the ion-exchange and neutralization reactions of equations 1 to 4 above to migrate from a deionization compartment into a concentration compartment or electrode compartment when applying current. Moreover, at this time, it is desirable for the ions to be removed that have been adsorbed into the ion exchangers to continuously migrate to an ion-exchange membrane between the deionization compartment and the concentration compartment on the solid phase (ion exchanger) without desorbing into the liquid phase. That is, in the deionization compartment, it is desirable for the cation exchanger contacting the cation-exchange membrane and the anion exchanger contacting the anion-exchange membrane to each be packed in as though they form a continuous phase.
Furthermore, in an electrical deionization apparatus in which ion exchangers are packed into compartments as described above, in each of the deionization compartments and/or concentration compartments having the ion exchangers packed therein, there exist places where the cation-exchange groups and anion-exchange groups contact with each other. At a place where a cation-exchange group and an anion-exchange group contact with each other in a deionization compartment in particular, water dissociation (equation 5) will occur under the steep potential gradientH2O→H++OH−  (5)and the ion exchangers in the deionization compartment will be regenerated by H+ ions and OH− ions produced by this water dissociation (water dissociation), whereby pure water of high purity can be obtained. For efficient deionization, it is thus desirable to make it such that there are many places where the anion exchanger and the cation exchanger contact with each other and water dissociation occurs. Furthermore, H+ ions and OH− ions produced by water dissociation regenerate ion exchange groups of adjacent cation and anion exchangers one after another. When continuing to apply current, there will come to be a local lack of counter-ions to the functional groups at the contacting places between the cation exchanger and the anion exchanger. The water in the vicinity of the functional groups will be dissociated, and H+ ions and OH− ions can be continuously supplied to the cation-exchange groups and the anion-exchange groups. Moreover, not only with water but also with nonelectrolyte such as alcohol, polarization and dissociation will occur under a strong electric field to form anions and cations which will be adsorbed into the functional groups, enabling removal from the water. It is thus desirable for the contact places between the anion exchanger and the cation exchanger (the places where water dissociation occurs) to be numerous and to exist throughout the whole of each deionization compartment in particular, and furthermore it is desirable for the anion exchanger and the cation exchanger to each be disposed as though they form a continuous phase from the contact places.
Furthermore, in recent years there has been a demand for pure water of yet higher purity, it being desired for the concentration of TOC (total organic carbon) components contained in treated water to be low. As to TOC components contained in treated water obtained from electrical deionization treatment, there are endogenous one, i.e. those originating from components eluted out from the ion exchangers packed into the electrical deionization apparatus, and exogenous one, i.e. those originating from TOC contained in the water to be treated. Many of the TOC components eluted out from the ion exchangers are unreacted monomers or uncross-linked polymer electrolytes that became attached to an ion exchanger during synthesis of the ion exchanger. These are gradually eluted out into the liquid phase upon washing by passing water, and it is desirable to make the ion exchangers have a structure such that this washing can be carried out in as short a time as possible. Moreover, it is desirable to eliminate a cross-linking reaction from the ion-exchanger synthesis process so that contamination of the ion exchangers with uncross-linked polymer electrolytes can be prevented. Regarding TOC components contained in the water to be treated, on the other hand, they can be removed by being ionized under the strong potential gradient between cation and anion exchange groups same as with the water dissociation reaction. It is thus desirable for it to be possible to supply water to be treated containing TOC components uniformly to the contact places between the cation exchanger and the anion exchanger.
Moreover, it is further desirable for the treated water (pure water) obtained to have a low concentration of weak electrolytes such as silica and carbonate. Again, ionization of such weak electrolytes under the strong potential gradient is effective same as with the water dissociation reaction between cation and anion exchange groups. Accordingly, it is also desirable for it to supply the contact places between the cation exchanger and the anion exchanger with water to be treated containing weak electrolytes uniformly.
The functions demanded for an electrical deionization apparatus have been described above; however, with a conventional electrical deionization apparatuses, it has not been possible to obtain an apparatus that satisfies all of these demands.
For example, with many conventional electrical deionization apparatuses, mixed anion-exchange resin beads and cation-exchange resin beads have been packed together in a deionization compartment. In this case, the state of packing of the resins is random, and moreover the flow direction of water in the compartment is also random, and from microscopic viewpoint, regarding the contact between the water to be treated and the ion exchangers, it is not necessarily the case that the water to be treated contacts the anion exchanger and the cation exchanger alternately. Moreover, regarding the particle diameter of the ion-exchange resin beads packed in, in general beads having a particle diameter of approximately 500 μm are used so as to decrease pressure loss, but most of the functional groups of such ion-exchange resin beads are present in macropores and micropores inside the resin beads, and hence it is difficult for ions to be removed to diffuse to the vicinity of functional groups, and hence the frequency of contact between the ions to be removed and the functional groups is not very high. Moreover, because the cation exchanger and the anion exchanger are packed in randomly, it is difficult for the cation exchanger and the anion exchanger to form a continuous phase, and hence it is difficult for the ions to be removed to migrate continuously through the solid phase from the deionization compartment into a concentration compartment, and moreover the TOC component removal performance and the weak electrolyte removal performance are poor. Furthermore, there has been a problem that there is much TOC components eluted from the ion-exchange resin beads, and in particular it is difficult to completely remove TOC components eluted from the inside of macropores and micropores even if the resin beads are washed with water for a long time.
Moreover, as to conventional electrical deionization apparatuses, method in which ion-exchange resin beads are packed in layers have also been proposed. As to electrical deionization apparatuses using this method, there are ones in which anion-exchange resin beads and cation-exchange resin beads are packed in layers alternately in a deionization compartment with a plastic mesh screen or the like interposed therebetween as necessary, ones in which a deionization compartment is divided with division plates and anion-exchange resin bead beds and cation-exchange resin bead beds are formed in each partition in the compartments, ones in which blocks are formed by binding ion-exchange resin beads with a binder and anion-exchange resin bead blocks and cation-exchange resin bead blocks are packed in alternately, and so on. However, packing anion-exchange resin beads and cation-exchange resin beads into a deionization compartment alternately in an orderly fashion while forming laminate structure is very difficult. Moreover, in the case that a mesh screen or the like is interposed between layers, or the deionization compartment is divided with division plates alternately, the contact plates between the anion and cation exchange groups (the places where water dissociation occurs) are limited to the planes contacting the ion-exchange membranes and the ion exchangers packed into the compartment, and hence it is not possible to form a large number of places where water dissociation occurs in the deionization compartment. Moreover, in the case that the deionization compartment is divided with division plates, the number of resin layers that can be formed is limited to from a few to a few dozens from the workability of assembling the compartment and the overall size of the apparatus. Moreover, in the case that ion-exchange resin beads are used, as described above, the frequency of contact between the ions to be removed and the ion exchange groups is not very high. Furthermore, in the case of binding ion-exchange resin beads with a binder, the flow of the water is restricted by the binder, and hence the frequency of contact between the ions to be removed and the ion exchange groups is lowered remarkably. Moreover, because ion-exchange resin beads are used, as described above, the much amount of TOC elutes, and in particular in the case of using a binder, the binder itself becomes a component that elutes, and hence the TOC concentration in the treated water becomes yet higher. Furthermore, as described above, the contact places between the anion and cation exchange groups are limited to the planes of contact between the ion-exchange membranes and the ion exchangers packed into the compartment, and most of the water to be treated does not flow through here, and hence ionization of TOC components and weak electrolytes is difficult, and thus the removal performance thereof is poor.
To resolve the various problems for causing use of ion-exchange resin beads described above, using an ion-exchange fibrous material obtained by introducing ion-exchange groups to a fibrous material such as a woven fabric or a nonwoven fabric by radiation-induced graft polymerization or the like as a material packed into a deionization compartment has been proposed (e.g. previously mentioned JP-A-H5-64726). Such an ion-exchange fibrous material has a greater specific surface area than ion-exchange resin beads, and it is not the case that the ion-exchange groups are present in micropores or macropores inside of ion exchange resin beads, but most of ion-exchange groups are disposed on the fiber surfaces. Therefore, the ions to be removed in the water to be treated are readily transported by the water flow (effect by convection) to the vicinity of the ion-exchange groups. When an ion-exchange fibrous material is used, comparing with the case of ion-exchange resin beads, the frequency of contact between the ions to be removed and the ion-exchange groups can thus be increased markedly.
However, a fibrous material such as woven fabric or a nonwoven fabric does not generally have high water permeability, and hence it has been thought that if a fibrous material is packed into a conventional narrow deionization cell, then the pressure loss will be too high to obtain a sufficient water flowrate.
An electrical deionization apparatus has thus been proposed in which a cation-exchange fibrous material such as a nonwoven fabric at the cation-exchange membrane side and an anion-exchange fibrous material at the anion-exchange membrane side are disposed face-to-face in a deionization compartment, and for an example an oblique-net spacer, or an ion-conducting spacer obtained ion-conductivity, is packed between these ion-exchange fibrous materials (e.g. the previously mentioned WO99/48820). In the case of an apparatus having such a constitution, the water to be treated flows with turbulent state inside the oblique-net spacer or ion-conducting spacer, and contacts the cation-exchange fibrous material and anion-exchange fibrous material. Although the water to be treated thus contacts the cation-exchange fibrous material and the anion-exchange fibrous material alternately, alternate contact is not carried out sufficiently efficiently with this constitution. Moreover, although fibrous materials having a large surface area and a lot of available ion-exchange groups are used, due to the difference in the water permeability between the fibrous materials and the spacer, most of the water to be treated flows through the spacer, and significantly little water flows through the nonwoven fabrics. The frequency of contact between the ions to be removed and the ion-exchange groups is thus low.